Study On The Humidity And Gas Sensing Of Surface Acoustic Wave Sensor Based On Bacterial Cellulose

Surface acoustic wave(SAW)sensors use SAW technology to convert the measured physical quantities into electrical signals that we need.Since most of the SAW energy is concentrated on the substrate surface,the sensor is very sensitive to the external environment and has very good sensitivity.Bacterial cellulose(BC)consists of ultra-fine filamentous fibers interwoven to form a developed ultra-fine network structure,and the surface contains a large number of pores and a large number of hydroxyl groups,which is very suitable as a sensitive membrane material for sensors.This paper studied the moisture-sensitivity properties of BC-based sensing membranes and their specific structures to improve the gas-sensitivity of polyethyleneimine(PEI)to formaldehyde gas.BC film's ultra-fine network structure,a large number of pores,and a large number of hydroxyl groups contained on its surface,make it have super strong water-permeability.Therefore,we investigated the moisture sensitive performance and sensing mechanism of SAW sensor based on BC sensitive film.The developed humidity sensor is environmentally friendly,low cost,and has superior sensitivity,fast response and fast recovery performance.When the relative humidity increases from 30%to 93%and then returns to 30%,the humidity sensor with the thickest BC film(148 nm)has a frequency shift of 89.8 kHz,the longest response time is 12 s,and the recovery time is 5s.Meanwhile,it also has excellent properties,such as excellent cross selectivity,good short-term repeatability and long-term stability.At normal temperature,PEI can selectively adsorb formaldehyde molecules through a reversible nucleophilic addition reaction.However,the surface of the PEI nanofilm is smooth,the specific surface area is low,and the PEI particles are agglomerated,and the detection performance of formaldehyde gas is not high.BC nanomembrane is composed of ultra-fine filament fibers interwoven with each other to form a developed ultra-fine network structure,which can provide a large number of attachment sites for PEI particles,and the hydroxyl group of BC and the amine group of PEI can form hydrogen bonds,so that The BC film surface is evenly distributed.Therefore,we investigated the use of BC nanofilms to improve the sensitivity of PEI nanofilms to formaldehyde gas.Experimental results show that the sensitivity of PEI/BC double-layer membranes to formalhyde gas is much higher than that of pure PEI membranes,especially at low concentrations.And its response frequency shift is related to the number of PEI film layers,formaldehyde concentration,and relative humidity.The sensor uses PEI/BC double-layer nanofilms with 3 PEI layers as the sensing layer,and has the best sensing performance.At room temperature(25~oC)and30%RH humidity,the sensor has a frequency shift of 35.6 kHz to 10 ppm formaldehyde gas,as well as good selectivity and stability.